This invention relates to the storage and transportation of compressed gases. In particular, the present invention includes methods and apparatus for storing and transporting compressed gas, a marine vessel for transporting the compressed gas and storage components for the gas, a method for loading and unloading the gas, and an overall method for the transfer of gas, or liquid, from one location to another using the marine vessel. More particularly, the present invention relates to a compressed natural gas transportation system specifically optimized and configured to a gas of a particular composition.
The need for transportation of gas has increased as gas resources have been established around the globe. Traditionally, only a few methods have proved viable in transporting gas from these remote locations to places where the gas can be used directly or refined into commercial products. The typical method is to simply build a pipeline and xe2x80x9cpipexe2x80x9d the gas directly to a desired location. However, building a pipeline across international borders is sometimes too political to be practical, and in many cases is not economically viable, e.g. where the gas must be transported across water, because deep water pipelines are extremely expensive to build and maintain. For example, in 1997, the proposed 750 mile pipeline linking Russia and Turkey via the Black Sea, was estimated to have an initial cost of 3 billion dollars, without any consideration for maintenance. In addition, costs are also increased because both construction and maintenance are treacherous and require extremely skilled workers. Similarly, transoceanic pipelines are not an option in certain circumstances due to their limitations regarding depth and bottom conditions.
Due to the limitations of pipelines, other transportation methods have emerged. The most readily apparent problem with transporting gas is that in the gas phase, even below ambient temperature, a small amount of gas occupies a large amount of space. Transporting material at that volume is often not economically feasible. The answer lies in reducing the space that the gas occupies. Initially, it would seem intuitive that condensing the gas to a liquid is the most logical solution. A typical natural gas (approximately 90% CH4) can be reduced to {fraction (1/600)}th of its gaseous volume when it is compressed to a liquid. Gaseous hydrocarbons that are in the liquid state are known in the art as liquefied natural gas, more commonly known as LNG.
As indicated by the name, LNG involves liquefaction of the natural gas and normally includes transportation of the natural gas in the liquid phase. Although liquefaction would seem the solution to the transportation problems, the drawbacks quickly become apparent. First, in order to liquefy natural gas, it must be cooled to approximately xe2x88x92260xc2x0 F., at atmospheric pressure, before it will liquefy. Second, LNG tends to warm during transport and therefore will not stay at that low temperature so as to remain in the liquefied state. Cryogenic methods must be used in order to keep the LNG at the proper temperature during transport. Thus, the cargo containment systems used to transport LNG must be truly cryogenic. Third, the LNG must be re-gasified at its destination before it can be used. This type of cryogenic process requires a large initial cost for LNG facilities at both the loading and unloading ports. The ships require exotic metals to hold LNG at xe2x88x92260xc2x0 F. The cost is generally in excess of one billion dollars for a full scale facility for one particular route for loading and unloading the LNG which often makes the method uneconomical for universal application. Liquefied natural gas can also be transported at higher temperatures than xe2x88x92260xc2x0 F. by raising the pressure, however the cryogenic problems still remain and the tanks now must be pressure vessels. This too can be an expensive alternative.
In response to the technical problems of a pipeline and the extreme costs and temperatures of LNG, the method of transporting natural gas in a compressed state was developed. The natural gas is compressed or pressurized to higher pressures, which may be chilled to lower than ambient temperatures, but without reaching the liquid phase. This is what is commonly referred to as compressed natural gas, or CNG.
Several methods have been proposed heretofore that are related to the transportation of compressed gases, such as natural gas, in pressurized vessels, either by marine or overland carriers. The gas is typically transported at high pressure and low temperature to maximize the amount of gas contained in each gas storage system. For example, the compressed gas may be in a dense single-fluid (xe2x80x9csupercriticalxe2x80x9d) state.
The transportation of CNG by marine vessels typically employs barges or ships. The marine vessels include in their holds, a multiplicity of closely stacked storage containers, such as metal pressure bottle containers. These storage containers are resistant internally to the high pressure and low temperature conditions under which the CNG is stored. The holds are also internally insulated throughout to keep the CNG and its storage containers at approximately the loading temperature throughout the delivery voyage and also to keep the substantially empty containers near that temperature during the return voyage.
Before the CNG is transported, it is first brought to the desired operating state, e.g. by compressing it to a high pressure and refrigerating it to a low temperature. For example, U.S. Pat. No. 3,232,725, hereby incorporated herein by reference for all purposes, discloses the preparation of natural gas to conditions suitable for marine transportation. After compression and refrigeration, the CNG is loaded into the storage containers of the marine vessels. The CNG is then transported to its destination. A small amount of the loaded CNG may be consumed as fuel for the transporting vessel during the voyage to its destination.
When reaching its destination, the CNG must be unloaded, typically at a terminal comprising a number of high pressure storage containers, pipelines, or an inlet to a high pressure turbine. If the terminal is at a pressure of, for example, 1000 pounds per square inch (xe2x80x9cpsixe2x80x9d) and the marine vessel storage containers are at 2000 psi, valves may be opened and the gas expanded into the terminal until the pressure in the marine vessel storage containers drops to some final pressure between 2000 psi and 1000 psi. If the volume of the terminal is very much larger than the combined volume of all the marine vessel storage containers together, the final pressure will be about 1000 psi.
Using conventional procedures, the transported CNG remaining in the marine vessel storage containers (the xe2x80x9cresidual gasxe2x80x9d) is then compressed into the terminal storage container using compressors. Compressors are expensive and increase the capital cost of the unloading facilities. Additionally, the temperature of the residual gas is increased by the heat of compression. This increases the required storage volume unless the heat is removed and raises the overall cost of transporting the CNG. Finally, and most importantly, because of the drop in pressure of the gas remaining in the marine vessel storage containers, the temperature in these containers will also drop, possibly below the safety limits of the container material. A related problem occurs when loading the gas into the marine containers, where instead of expansion causing cooling as above, compression of the injected gas by later injections causes it to heat, thus raising the temperature above the targeted storage conditions.
Previous efforts to reduce the expense and complexity of unloading CNG, and the residual gas in particular, have introduced problems of their own. For example, U.S. Pat. No. 2,972,873, hereby incorporated herein by reference for all purposes, discloses heating the residual gas to increase its pressure, thereby driving it out of the marine vessel storage containers. Such a scheme simply replaces the additional operating cost associated with operating the compressors with an operating cost for supplying heat to the storage containers and residual gas. Further, the design of the piping and valve arrangements for such a system is necessarily extremely complex. This is because the system must accommodate the introduction of heating devices or heating elements into the marine vessel storage containers.
In summary, although CNG transportation reduces the capital costs associated with LNG, the costs are still high due to a lack of efficiency by the methods and apparatus used. This is due primarily to the fact that prior art methods do not optimize the vessels and facilities for a particular gas composition. In particular, prior art apparatus and methods are not designed based upon a specific composition of gas to determine the optimum storage conditions for a particular gas.
U.S. Pat. No. 4,846,088 discloses the use of pipe for compressed gas storage on an open barge. The storage components are strictly confined to be on or above the deck of the ship. Compressors are used to load and off load the compressed gas. However, there is no consideration of a pipe design factor and no attempt to obtain the maximum compressibility factor for the gas.
U.S. Pat. No. 3,232,725 does not contemplate a specific compressibility factor to then determine the appropriate pressure for the gas. Instead, the ""725 patent discloses a broad range or band to get greater compressibility. However, to do that, the gas container wall thickness will be much greater than is necessary. This would be particularly true when operated at a lower pressure causing the pipe to be over designed (unnecessarily thick). The ""725 patent shows a phase diagram for a mixture of methane and other hydrocarbons. The diagram shows an envelop inside which the mixture exists as both a liquid and a gas. At pressures above this envelop the mixture exists as a single phase, known as the dense phase or critical state. If the gas is pressured up within that state, liquids will not fall out of the gas. Also, good compression ratios are achieved in that range. Thus, the ""725 patent recommends operation in that range.
The ""725 patent graph is based on the lowering of temperatures. However, the ""725 patent does not design its method and apparatus by optimizing the compressibility factor at a certain temperature and pressure and then calculating the wall thickness needed for a certain gas. Since much of the capital cost comes from the large amount of metal, or other material, required for the pipe storage components, the ""725 misses the mark. The range offered in the ""725 patent is very broad and is designed to cover more than one particular gas mixture, i.e., gas mixtures with different compositions.
U.S. Pat. No. 4,446,804 discloses offloading using a displacing fluid. The ""804 patent does not consider low temperature fluids as the oil and gas are taken directly from a producing well and extreme temperatures are not considered. It also does not consider onshore storage or thermal shock caused by liquids or gases upon containers of different temperatures. Thermal shock occurs when a material is suddenly exposed to an extreme temperature change, causing severe local stresses. It is the reason LNG facilities require a cool down period before being exposed to full LNG flow. The ""804 patent carries the displacement fluid on the vessel which is used to displace sequential tanks. No mention is made of low temperature requirements.
The present invention overcomes the deficiencies of the prior art by providing a method for optimizing a transportation vessel for compressed gas; the design of that transportation vessel and design of the storage components for the gas aboard that vessel; a method for loading and unloading the gas; and an overall method for the transfer of gas from one location to another using the optimized transportation vessel; as well as specific apparatus for use with the methods.
The methods and apparatus of the present invention for transporting compressed gas includes a gas storage system optimized for storing and transporting a compressible gas. The gas storage system includes a plurality of pipes in parallel relationship and a plurality of support members extending between adjacent tiers of pipe. The support members have opposing arcuate recesses for receiving and housing individual pipes. Manifolds and valves connect with the ends of the pipe for loading and off-loading the gas. The pipes and support members form a pipe bundle which is enclosed in insulation and preferably in a nitrogen and enriched environment.
The gas storage system is optimized for storing a compressible gas, such as natural gas, in the dense phase under pressure. The pipes are made of material which will withstand a predetermined range of temperatures and meet required design factors for the pipe material, such as steel pipe. A chilling member cools the gas to a temperature within the temperature range and a pressurizing member pressurizes the gas within a predetermined range of pressures at a lower temperature of the temperature range where the compressibility factor of the gas is at a minimum. The preferred temperature and pressure of the gas maximizes the compression ratio of gas volume within the pipes to gas volume at standard conditions. The compression ratio of the gas is defined as the ratio between the volume of a given mass of gas at standard conditions to the volume of the same mass of gas at storage conditions.
As for example, one preferred embodiment of the gas storage system includes pipes made of X-60 or X-80 premium high strength steel with the gas having a temperature range of between xe2x88x9220xc2x0 F. and 0xc2x0 F. The lower temperature in the range is xe2x88x9220xc2x0 F. For X-100 premium high strength steel, the lower temperature may be negative 40xc2x0 F. For a gas with a specific gravity of about 0.6, the pressure range is between 1,800 and 1,900 psi and for a gas with a specific gravity of about 0.7, the pressure range is between 1,300 and 1,400 psi. The range of pressures at the lower temperature is the pressure range where the compressibility factor varies no more than two percent of the minimum compressibility factor for a gas with a particular specific gravity.
Once the strength of the steel and the pipe diameter are selected, for a given design factor, the pipe wall thickness is determined by maximizing the ratio of the mass of the stored gas to the mass of the steel pipe. By way of further example, for a gas with a specific gravity of substantially 0.6 and where the design factor is one-half the yield strength of the steel pipe having a yield strength of 100,000 psi and a pipe diameter of 36 inches, the pipe wall thickness will be between 0.66 and 0.67 inches. For a gas with a specific gravity of substantially 0.7 in the above example, the pipe wall thickness will be between 0.48 and 0.50 inches.
The wall thickness of the pipe may be increased by adding an additional thickness of material for a corrosion or erosion allowance. This thickness is above the thickness required to maintain the resultant yield stress. This allowance may be as much as 0.063 inches or greater depending on the application. The large diameter pipe used in the current invention allows this allowance to be incorporated without unacceptable degradation of the system efficiency. Although the preferred embodiment of the present invention uses high strength carbon steel pipe, other materials may find application in this system. Materials such as stainless steels, nickel alloys, carbon-fiber reinforced composites, as well as other materials may provide an alternative to high strength carbon steel.
The present invention is particularly directed to methods and apparatus for transporting compressed gases on a marine vessel. Preferably the gas storage system on the marine vessel is designed for transporting a gas with a particular gas composition. Where the gas to be transported varies from the design gas composition for the gas storage system, a gas of a second gas composition may be added or removed from the gas to be transported until the resultant gas has the same gas composition as the particular gas composition for which the gas storage system is designed.
The gas storage system may be an integral part of the marine vessel. The marine vessel may include a hull having a support structure with the pipes of the gas storage system forming a portion of the support structure. The hull may be divided into compartments each having a nitrogen atmosphere with a chemical monitoring system to monitor for gas leaks. A flare system may also be included to bleed off any leaking gas. The hull is insulated preventing the temperature of the gas from raising more than xc2xdxc2x0 per 1,000 miles of travel of the marine vessel. As an alternative, the marine vessel may include a hull constructed from concrete with gas storage pipes built into the hull section. A bow section is connected to one end of the hull section and a stern section is connected to the other end of the hull section.
The gas storage system may be built as a modular unit with the modular unit either being supported by the deck of the marine vessel or being installed within the hull of the marine vessel. The pipes in the modular unit may extend either vertically or horizontally with respect to the deck.
The stored gas is preferably unloaded by pumping a displacement fluid into one end of the gas storage system and opening the other end of the gas storage system to enable removal of the gas. A displacement fluid is selected which has a minimal absorption by the gas. A separator may be disposed in the gas storage system to separate the displacement fluid from the gas to further prevent absorption. Preferably, the gas is off-loaded one tier of pipes at a time. The gas storage system may also be tilted at an angle to assist in the off-loading operation.
The method of transporting the gas includes optimizing the gas storage system on the marine vessel for a particular gas composition for a gas being produced at a specific geographic location. The system includes a loading station at the source of the natural gas and a receiving station for unloading the gas at its destination. The gas storage system is optimized at a pressure and temperature that minimizes the compressibility factor of the gas and maximizes the storage efficiency ratio of the system.
Although the present invention is particularly directed to methods and apparatus for transporting compressed gas, it should be appreciated that the embodiments of the present invention are also applicable to transporting liquids such as liquid propane.
The embodiments of the present invention provide many unique features including but not limited to:
a) Structural integration of a gas storage system with a marine vessel to structurally stiffen the marine vessel, with the storage system including supports serving as bulkheads, the storage system components serving as bulkheads, the gas storage system serving as buoyancy, and the storage system providing storage of all gases and liquids;
b) Construction of a gas storage system as a containerized system allowing the transport of the system on the deck, or in the hull, of a marine vessel wherein the gas storage system is essentially independent of the structure of the marine vessel;
c) Staged loading and off-loading using low freezing point liquid stored either on-shore or on the marine vessel;
d) Loading and off-loading using liquid driven pigs to separate the gas from the liquid;
e) Matching of gas storage pipe dimensions, such as diameter and wall thickness, to the optimized compressibility factor for the composition of a defined gas supply so as to minimize the weight of the steel per unit weight of stored gas on the vessel;
f) Use of premium pipe, manufactured to accepted standards, such as API, ASME, or class society rules, as storage on a marine vessel with a design factor higher than that for individually built pressure vessels, i.e., the design factor being higher than 0.25 or similar standard;
g) Insulation lining of entire hull or the assembly of containers, reducing temperature rise to an acceptable rate for the desired service, such as less than one degree per 100 hours of travel;
h) Trimming of a marine vessel, or tilting of a gas storage system, in order to decrease surface contact area between gas cargo and displacement liquid and maximize the evacuation of displacement liquid from the gas storage system;
i) Taking pressure drop across control valve during the off-loading phase either on-shore or on the vessel but outside of the primary gas containers, thereby avoiding a temperature drop in these containers;
j) Use of manifolding to isolate the specific pipes of a gas storage system most prone to damage, such as the sides and bottom of the vessel, from external causes;
k) Hydrostatic testing during liquid displacement; and
l) Method of construction of a marine vessel.
An advantage of the present invention is that the high capital costs and cryogenic procedures normally associated with transporting natural gas across water may be significantly reduced making the profitability of the present invention greater than previously used methods and apparatus.
The present invention includes improvement of CNG storage and transportation methods and apparatus, by optimizing the CNG storage conditions, thereby overcoming the deficiencies of the prior methods of natural gas storage and transportation.
Other objects and advantages of the invention will appear from the following description.